On Tue, May 01, 2018 at 04:31:14PM -0500, Rob Herring wrote: > Deferred probe will currently wait forever on dependent devices to probe, > but sometimes a driver will never exist. It's also not always critical for > a driver to exist. Platforms can rely on default configuration from the > bootloader or reset defaults for things such as pinctrl and power domains. > This is often the case with initial platform support until various drivers > get enabled. There's at least 2 scenarios where deferred probe can render > a platform broken. Both involve using a DT which has more devices and > dependencies than the kernel supports. The 1st case is a driver may be > disabled in the kernel config. The 2nd case is the kernel version may > simply not have the dependent driver. This can happen if using a newer DT > (provided by firmware perhaps) with a stable kernel version. > > Unfortunately, this change breaks with modules as we have no way of > knowing when modules are done loading. One possibility is to make this > opt in or out based on compatible strings rather than at a subsystem level. > Ideally this information could be extracted automatically somehow. OTOH, > maybe the lists are pretty small. There's only a handful of subsystems > that can be optional, and then only so many drivers in those that can be > modules (at least for pinctrl, many drivers are built-in only). > > Cc: Alexander Graf <agraf@xxxxxxx> > Signed-off-by: Rob Herring <robh@xxxxxxxxxx> > --- > This patch came out of a discussion on the ARM boot-architecture > list[1] about DT forwards and backwards compatibility issues. There are > issues with newer DTs breaking on older, stable kernels. Some of these > are difficult to solve, but cases of optional devices not having > kernel support should be solvable. > > I tested this on a RPi3 B with the pinctrl driver forced off. With this > change, the MMC/SD and UART drivers can function without the pinctrl > driver. > > Rob > > [1] https://lists.linaro.org/pipermail/boot-architecture/2018-April/000466.html > > drivers/base/dd.c | 16 ++++++++++++++++ > drivers/pinctrl/devicetree.c | 2 +- > include/linux/device.h | 2 ++ > 3 files changed, 19 insertions(+), 1 deletion(-) > > diff --git a/drivers/base/dd.c b/drivers/base/dd.c > index c9f54089429b..5848808b9d7a 100644 > --- a/drivers/base/dd.c > +++ b/drivers/base/dd.c > @@ -226,6 +226,15 @@ void device_unblock_probing(void) > driver_deferred_probe_trigger(); > } > > + > +int driver_deferred_probe_optional(void) > +{ > + if (initcalls_done) > + return -ENODEV; > + > + return -EPROBE_DEFER; > +} The name is ackward for this function, but I can't think of anything better at the moment, sorry. However, the overall idea for this is sane, no objection from me at all for this change. thanks, greg k-h -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
